MOND is really a rule without a theory; it simply posits that there is an acceleration cutoff below which gravity does not obey an inverse square law. This conjecture has been shown to have impressive predictive power, and it naturally explains a bunch of observations that are not obviously related and require fine-tuning in a dark matter paradigm. The blog linked here goes into a lot of detail on those observations, as well as observed phenomena that don't seem to fit well with MOND.

What the theory underlying MOND actually is is anyone's guess, and an area of active conjecture. But that's just as true for dark matter.